How can I do color keying using the VSE?
Option 1

The VSE cannot do a chroma key by itself. You can however make a simple key with modifiers. In this example I have:


The foreground strip (source A) at the bottom to serve as key source
Then I slightly blur it to hide imperfect edges





The yellow color effect serves as background
Duplicate the foreground source clip to the top (source B)
Add a Mask Modifier to it using strip Blur as its input





To source A add a Hue Correct modifier and set its Value and
increase key colour (blue or green) range to all other colours to
0 . Use the curve to tune colours in or out of key. Switch to the S button for saturation and edit the curve so that all Control Points are at 0 or black and white.
Add Curve modifier to source A. Raise the black control point
(bottom left to the top and lower the white point control point (top
right) to the bottom. Place an extra control point in the middle and drag it right up to the top making a high contrast key image.


Remember to press the refresh Sequencer button while you do these changes to see the result.

Option 2:

Otherwise use the compositor. It has proper keying nodes. You can add your source clip as an input to a compositor in another scene (this is important). Perform the key there but don't add a background, make sure to use the Convert Alpha node to premultiply the output. In the VSE add that scene (with the compositor active) as a scene strip, make sure to use Alpha Over. Then place and trim as you wish over the background strip in the VSE.